The present invention relates to a structure and driving method for a picture display device using a liquid crystal.
FIG. 1 shows a circuit structure for a conventional picture display device to display a picture in accordance with analogue signals, the picture display device being comprised of a liquid crystal and an MOS-FET array. A unit picture element comprises a MOS-FET 1, a storage capacitor 2, and a liquid crystal cell 3. The fundamental operation of the picture display device will be illustrated. When a negative pulse voltage is applied to a gate line x.sub.i as a gate signal, the P-channel MOS-FET 1 is in an ON state and analogue video signals applied to a signal line y.sub.i are charged in the storage capacitor 2 via the FET 1. The FET 1 is in an OFF state if the negative pulse signal is terminated, however, since a leak current through the FET 1 and a current through the liquid crystal cell 3 are exceedingly small in general, the voltage charged in the storage capacitor 2 is maintained for a considerably long time and kept applied to the liquid crystal cell 3. The gate signals are scanned from x.sub.i to x.sub.i +1, x.sub.i +2 . . . in a line sequence, and the video signals corresponding to the position of the gate signals are applied from y.sub.i, y.sub.i+1, y.sub.i+2 . . . , whereby the overall picture is displayed.
FIG. 2 shows a sectional view of a picture element including the FET. In the case of the P-channel FET, numeral 4 denotes an n-type Si substrate, 5 and 6 respectively denote a source and drain in the P.sup.+ diffusion region. The source 5 is connected in the y.sub.i direction in FIG. 1. Numeral 7 denotes a gate oxide, and 8 denotes a gate electrode connected in the x.sub.i direction. Numeral 9 denotes a picture element electrode of the liquid crystal cell forming one electrode, and a condensor is formed between a thin oxide 11 and the substrate 4. Numeral 12 denotes a liquid crystal, and 13 denotes a transparent counter electrode which functions as a common electrode for the overall picture area. Numeral 14 denotes a glass substrate and 10 denotes an insulating layer. In this structure, the voltage polarity applied to the liquid crystal is in one direction for direct-current drive. Accordingly this structure is disadvantageous in reliability, e.g., the life of the liquid crystal is short. In order to eliminate the above noted drawbacks, the following proposal has been made. Namely, the common electrodes of the liquid crystal cells are grouped in common lines as shown in FIG. 3, and the gate signal lines x.sub.i, x.sub.i+1, x.sub.i+2 . . . are synchronized with the common electrode lines z.sub.i, z.sub.i+1, z.sub.i+2 to be scanned in turn, and the video signals and the common electrode electric potentials are simultaneously inverted for expecting an alternating drive (Japanese Laid Open Patent No. 120095/80). In this method, however, it is necessary to connect the common electrode with the circuit by dividing it into 240 in order to display 240.times.240 picture elements, whereby the mounting of the common electrode is very difficult.